Poison ivy and stinging nettles can cause a lot of pain, but they have nothing on the Gympie-Gympie tree, whose leaves can put people into intensive care and induce agony that lasts for days or even months. There are unverified accounts of animals needing to be put down after close encounters with Gympie-Gympie trees, but the exploration of its toxins has only just begun. The first study surprised scientists, revealing these trees produce molecules more like certain animal venoms than known plant defenses.
Predictably, Gympie-Gympie trees come from Australia, where there is even a town named after them. They're one of several stinging trees of the Dendrocnide genus found in Queensland but pose a particular menace by being common close to more heavily populated areas like the Gold Coast hinterland, rather than restricted to the tropics. Dr Irina Vetter of the University of Queensland told IFLScience related species are known from Thailand and the Philippines. These have yet to be studied, so Australia's proud tradition of having the most venomous specimens of everything currently extends to trees.
“Like other stinging plants such as nettles, the giant stinging tree is covered in needle-like appendages called trichomes that are around five millimetres in length – the trichomes look like fine hairs, but actually act like hypodermic needles that inject toxins when they make contact with skin,” Vetter said in a statement.
However, the pain is much more long-lasting because instead of histamines and formic acid used by temperate zone plants, the Gympie-Gympie use neurotoxic proteins.
In Science Advances, Vetter reports that these neurotoxins are a new class of miniproteins, naming them gympietides after the tree.
“Although they come from a plant, the gympietides are similar to spider and cone snail toxins in the way they fold into their 3D molecular structures and target the same pain receptors,” she said.
This is apparently a rare example of convergent evolution spanning the animal and plant kingdoms. The pain they induce appears to be a result of altering sodium channels in the neurons of animals that touch their leaves, which can take a long time to recover.
This means understanding gympietides could have implications far beyond easing the pain of those who are stung. Working out how these molecules produce such long-lasting effects could help us come to grips with the still-mysterious nature of chronic pain.
“We might be able to design molecules that block pain signals and therefore make better painkillers” for lasting pain, Vetter told IFLScience.
Vetter still does not know why Gympie-Gympie and their relatives developed such powerful neurotoxins. Some have speculated they act as insecticides, but Vetter is skeptical having seen leaves thoroughly devoured by beetles. A defense against herbivorous vertebrates makes more sense, but Vetter says there are rumors of pademelons eating the leaves. If so, this would induce a whole new respect for these absurdly cute members of the kangaroo family since, as Vetter said, “I wouldn't be putting my mouth near [the trees].”